1. Field of Invention
This invention relates in general to micro-actuators and, more particularly, to a bistable switch at least partially constructed of shape memory alloy (SMA).
2. Description of the Related Art
The first electro-mechanical and solid state micro-switches were developed in the late 1940's. The importance of micro-switch technology has increased as the trend toward miniaturization of electrical components developed. Current electro-mechanical micro-switches are expensive and complex to construct. Moreover, the components of current electro-mechanical micro-switches tend to be susceptible to mechanical breakdown. Solid state micro-switches are characterized by high on-state resistance, and, for many applications, undesirably high on-state “contact” coupling capacitance.
Another trend in the area of electronic switches has been to utilize SMA's to perform switching functions. A SMA material is a specialized alloy that exhibits a given mechanical movement in response to heating above a threshold temperature. The movement is relatively precise, predictable, and repeatable. When the SMA material is allowed to cool below the threshold temperature, it attains a ductile state. The SMA material is chiefly characterized by this ability to undergo reversible transformations between a first conformation at a sub-threshold temperature and a second conformation at a temperature above the threshold.
U.S. Pat. No. 4,887,430 to Kroll et al. describes a bistable shape memory alloy (SMA) actuator having separate first and second SMA elements that move an actuator along a travel stroke between first and second positions. The Kroll actuator selectively heats the first and second SMA elements to move the actuator between the first and second positions. The actuator employs a mechanical frictional retainer to bias the actuator in its first and second positions.
The Kroll invention can be utilized to provide bistable mechanical actuation. However, the means for biasing the transducer or actuator in its first and second positions is mechanically separate and distinct from the element being actuated. Additionally, the Kroll actuator employs two or more separate SMA wires to provide movement between the first and second positions. Consequently, the Kroll device is not well suited for micro-switch designs that require the minimum number of components.
What is needed is a bistable SMA switch that is suitable for use as a micro-switch that is inexpensively manufacturable.